linux-kernel - Re: [PATCHv7 01/33] ns: Introduce Time Namespace

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Date:   Thu, 17 Oct 2019 10:47:36 +0100
From:   Vincenzo Frascino <vincenzo.frascino@....com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Andrei Vagin <avagin@...il.com>
Cc:     Dmitry Safonov <dima@...sta.com>, linux-kernel@...r.kernel.org,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Andrei Vagin <avagin@...nvz.org>,
        Adrian Reber <adrian@...as.de>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Jann Horn <jannh@...gle.com>, Jeff Dike <jdike@...toit.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        Shuah Khan <shuah@...nel.org>,
        containers@...ts.linux-foundation.org, criu@...nvz.org,
        linux-api@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCHv7 01/33] ns: Introduce Time Namespace

On 10/17/19 10:20 AM, Thomas Gleixner wrote:

[...]

> The architectures which implement VDSO are:
> 
>     arm, arm64, mips, nds32, powerpc, riscv, s390, sparc, x86, um
> 
> arm64, mips, x86 use the generic VDSO. Patches for arm are floating
> around. UM is special as it just traps into the syscalls. No idea about the
> rest. Vincenzo might know.
> 

There a couple of cases: hexagon and csky that have vDSOs for signal trampolines
if I recall correctly, but they do not fall into the category we are exploring
at the moment.

> The bad news is that we have no information (except on arm which has a
> config switch for VDSO) whether an architecture provides VDSO support or
> not.
> 
> So unless you add something like
> 
>    config HAS_VDSO
>    	  bool
> 
> which is selected by all architectures which provide VDSO support, the only
> sane solution is to depend on GENERIC_VDSO_TIME_NS.
> 
> TBH, I would not even bother. The architectures which matter and are going
> to use time namespaces already support VDSO and they need to move to the
> generic implementation anyway as we discussed and agreed on in Vancouver.
> 
> Providing time name spaces for the non VDSO archs is a purely academic
> exercise.

I totally agree with this.

-- 
Regards,
Vincenzo